The present invention relates generally to applied optics, spectra-chemical image processing, chemical identification, and chemical analysis. More specifically, the present invention relates to apparatus and methods for the non-destructive testing and identification of the composition of a sample of plastics or other materials using Raman spectroscopy and computerized signal processing.
A widespread need exists in industry and government for identifying materials. For example, automobile companies and plastics manufactures must identify and separate plastic resins in recycling operations. Pharmaceutical companies must monitor chemical constituents during drug production. Monitoring agencies and firms must assay waste stream flows into the environment. Law enforcement agencies must identify the presence of illicit drugs in the field in order to combat criminal drug trafficking.
In today""s environmentally conscious society, simple economics provides a strong incentive for manufacturers to minimize their use of natural resources and public landfills. Substantial economic benefit can be gained by turning to recycling as a source for raw materials and as the ultimate repository for the manufactured goods. Using recycled raw materials in products can increase profits by saving materials costs and energy. Efficiently recycling packaging and production waste can save landfill charges and provide a cost recovery stream. Further, manufacturing goods with recycled content, and designing goods that themselves are recyclable, is a civic duty that also offers public relations benefits that are worthwhile from a marketing standpoint.
Many suppliers, however, face difficulties in using recycled feed streams. All companies face competition, and in the marketplace, price alone does not guarantee market share. Most manufacturers value quality and consistency of goods more than the abstract notions of civic duty and environmental policy. Further, many production lines operate with just-in-time inventories, in which a factory receives all the components necessary to assemble a product only hours before they are needed. The presence of only a few defective parts can shut down a production line until replacement parts arrive. Such a shut down can cost a manufacturer many thousands of dollars.
Thus, it is easy to understand why companies have been reluctant to include recycled materials in products. Recycled materials must have documented histories so that they are assured of compatibility with the manufacturing process. A misidentified piece of recycled material included with virgin material can destroy an entire production run. Maintaining the history of recycled goods, or even knowing their exact composition is difficult, if not impossible, with current technology.
Millions of tons of plastic and other materials are deposited in landfills or incinerated every year due almost solely to the lack of sufficient technology to avoid cross contamination between different types of plastic or other material during collection. The need therefore exists for an effective, economical means to identify a variety of materials, and-specifically plastics, on site in scrap yards, warehouses, factories and recycling centers. The successful commercialization of an instrument with such capabilities would greatly increase the recycling rates for plastics and perhaps many other materials. By offering a simple means to overcome the difficult problem of material identification, the present invention seeks to help make manufacturers more receptive to including recycled content in their products, and purchasers more confident of the quality of those products.
Many methods exist for identifying materials. One test for plastic materials, for example, involves the burning of a small sample of the plastic material. Upon smelling the smoke, a trained technician can identify several different classes of plastics with reasonable success. While this method can be employed in a laboratory, such methods are not appropriate or practical for commercial or production line applications. This type of chemical analysis would also not be acceptable to law enforcement personnel or the courts for the identification of Cocaine.
An assortment of analytical identification methods exist, such as Fourier Transform Infrared Spectroscopy (FTIR) and X-ray fluorescence (XRF), for the non-destructive testing of materials. An example of FTIR technology is disclosed in U.S. Pat. No. 5,510,619. While well known and used, the FTIR process is not practical in many commercial applications because the method is very sensitive to dirt, surface roughness, coatings, moisture, and sample motion during identification. The XRF process is also used but it is relatively expensive. Other analytical identification methods are disclosed, for example, in U.S. Pat. No. 5,256,880 and 5,512,752.
Raman spectroscopy, discovered by C. D. Raman in 1928, has many unique qualities that can be advantageously employed in the practical identification of materials. Raman signals, generated by the interaction of monochromatic light and a sample, are not affected by dirt, surface finish, coatings, or any motion of the sample being identified. Significantly, Raman signals are also not as sensitive to water, glass or quartz as other infrared signals. As a result, chemical samples can be contained within a glass vessel, or even suspended in an aqueous solution without affecting the Raman signal. The Raman process also has a significantly higher depth of field than other processes and can xe2x80x9clook throughxe2x80x9d a container to the chemical sample contained inside.
Despite these advantages, Raman spectroscopy is not widely used because of a low signal to noise ratio inherent in Raman Spectroscopy. Traditionally, the excitation light source, typically a laser, is directed continuously against a chemical sample, and the Raman signal is collected over time. An example of such an apparatus is disclosed in U.S. Pat. No. 5,534,997. Increasing the power of the excitation laser in the Raman process can increase the strength of the Raman signal and reduce the required sampling time. However, the increase in power can cause thermal damage to the sample particularly if the sample has low thermal conductivity that is typical of plastics. The increase in power can also cause xe2x80x9cblack bodyxe2x80x9d or thermal radiation that can overwhelm the Raman signal. It is commonly assumed, therefore, that Raman spectroscopy is not appropriate for the practical identification of highly energy absorbent materials such as black or highly pigmented plastics. In the extreme case, such highly absorbent materials can char or burn thus rendering the material unsuitable for further use.
What is needed is a system for analyzing and identifying the composition of a wide variety of materials that is fast enough for practical application in a commercial setting, insensitive to sample impurities or surface imperfections, tolerant of water and common sample containers, and which does not damage the sample being analyzed and identified.
A system satisfying these needs generally comprises a probe including a housing having an optical window through which visible and infrared light can pass. A monochromatic light source is provided with radiation optics optically coupling the light source to the optical window so that light emitted from the light source is directed through the window toward a sample causing the sample to produce a characteristic Raman signal. Sampling optics are coupled to the optical window to receive the characteristic Raman signal produced from the sample including at least one filter for removing unwanted spectral portions. A spectrograph is coupled to the sampling optics for dispersing the characteristic Raman signal into a spectrum to form a spectrographic output. An optical detector is coupled to the spectrograph to receive the spectrographic output and generate a digital map representing the Raman spectrum as a function of wavelength. A computer is coupled to the detector that includes an input to receive the digital map, a library of digital maps of specimens of known composition, a comparison means for comparing a received digital map with the contents of the library to select any match exceeding a specified confidence level, and an output display of the name of the selected match. A trigger circuit is included that initiates collection of the spectrographic output by the detector at the same time or slightly before it initiates emission by the light source so that data concerning the characteristic Raman signal from the sample can be collected by the detector before any significant heating of the sample can take place. The trigger circuit initiates selectively one or more pulses of light from the light source of specified duration and power so as to avoid or reduce the black body or thermal radiation problem commonly experienced with other apparatus. The apparatus is suitable for use to identify a wide variety of materials in gas, liquid, solid, or powder form, including polymers, plastics, ceramics, minerals, composites, pharmaceuticals, petrochemicals, organics, inorganics, biochemicals, and organo-metallics.
The present invention advantageously employs one or more discrete pulses of light, preferably generated by a laser, to cause the Raman signal to be produced by a sample. The monochromatic light source preferably comprises an inexpensive near-infrared multimode laser operable in a pulse mode between a lower and an upper intensity. The lower intensity level is at or just above the threshold intensity below which laser activity discontinues, typically less than about 105% of the lasing threshold. The upper intensity can be at any level at or below a maximum intensity established by the upper power limit of the laser. A xe2x80x9clapse timexe2x80x9d or xe2x80x9coff timexe2x80x9d between the laser pulses is provided to allow the sample to dissipate localized heat generated by the laser pulse. Advantageously, a single location on a sample can be exposed to more than one pulse during the sampling process, and allowed to cool between pulses. The laser pulses are cycled repeatedly until an adequate Raman signal is collected to allow the required analysis to identify the sample. The pulse width defining the duration of the upper intensity can be between about 0.01 and 10.0 seconds, depending upon the nature of the sample. In the preferred embodiment employing a 1.2 Watt laser, the pulse duration is between about 80 and 120 milliseconds for materials sensitive to optical damage. For less sensitive materials, a typical pulse duration can be up to about 500 milliseconds. The pulse duration can be shortened by using a correspondingly higher power laser to deliver between about 0.05 to 0.5 joules of photon energy to the sample. The number and duration of pulses employed to test a given sample is controlled to provide a satisfactory Raman signal while minimizing the thermal strain on the sample and avoiding detector saturation at any given pixel position. Shorter pulses and multiple discrete pulses are an important feature of the present invention which advantageously provides an enhanced Raman signal to noise ratio, even when considering sources of the same power, as the thermal noise and likelihood of detector saturation is kept to a minimum.
In a preferred embodiment of the invention, the probe housing includes a handle to permit manipulation of the probe, and further comprises a trigger situated on the handle and coupled to the light source so that depression of the trigger temporarily increases the output of light from the source, preferably in the form of one or more prescribed pulses as previously described. The apparatus preferably includes a console for housing the spectrograph, the optical detector, and the computer, and the sampling optics include a fiber-optic umbilical coupled between the console and the probe. The light source is housed in the console and the radiation optics includes at least one optical fiber carried by the fiber-optic umbilical. The probe housing preferably comprises an elongated rail having a lower and an upper surface, a longitudinal axis of the probe being separated by a fixed distance from the upper surface. A plurality of supports are fixed to the rail upper surface for supporting optical elements to intersect the longitudinal axis of the probe. The probe housing also includes a tubular member including a longitudinal slot that receives the rail so that an axis of revolution of the tubular member is coincident with the longitudinal axis of the probe and parallel to the rail upper surface. A back plate closes one end of the tubular member while a nose cone containing the optical window closes the other end. An off-axis baffling tube is supported by the rail parallel to the longitudinal axis to define a specific segregated region within the probe to permit extinction of unwanted portions of the light source spectrum immediately prior to illumination of the sample. This design allows for an ease of construction that contributes to lowering the cost of the overall system sufficiently to permit widespread adoption of the system by both industry and government.
In the preferred embodiment, the optical elements held by the plurality of supports within the probe can include lenses providing the probe with a prescribed depth of field, at least one filter, and a reflective element inclined with respect to the longitudinal axis of the probe for aligning the laser output with the optical window. To achieve optimum Raman signals, the lenses uniformly distribute the laser output over a sample irradiation area so as to achieve a power density of between about 1 mW/mm2 and 1 W/mm2. This is achieved by employing lenses having an f-number of between about 1 and 3. Preferably, the sampling optics includes a plurality of optical fibers, each of the fibers including an entrance end and an exit end. The entrance ends of the optical fibers are situated in a compact array in a common plane within the probe housing so as maximally to collect the Raman signal produced by the sample. The exit ends of the optical fibers are arranged in a line at an entrance to the spectrograph that typically contains a diffraction grating, the ruled surface of which is aligned to be parallel with the line defined by the exit ends of the optical fibers. This feature has the advantage of providing a signal to the spectrograph that is of enhanced intensity over that which might be gathered by a single fiber yet without diminishing the spectrographic resolution. The optical detector comprises an array detector having a number of outputs sufficient to define a division of the spectrographic output into a set of discrete spectral elements having a pixel width of about 0.2 nm.
A laser particularly suitable for use with the present invention is a near-infrared (NIR) multi-mode diode laser, the wavelength and deviation of which are compatible with spectrographic identification apparatus. The use of such a laser in the present invention provides an excitation source of lower cost yet higher power and reliability than comparable single-mode diode laser. In the preferred embodiment, the laser is a 1.2 Watt, 798 nm pulsed multi-mode laser excitation source having a wavelength deviation of less than 7 nm and more typically about 1-3 nm. The use of an excitation source having a wavelength deviation significantly greater than the pixel width in the spectrographic output achieves the surprising result of simplifying the smoothing the spectrum detected by the discrete spectral elements without loss of spectral information. This smoothing function is particularly important when derivatives of the spectrum are used for comparison as the difference in wavelength permits a significant reduction in pixel noise. The ratio of excitation source wavelength deviation to spectral detection element width is desirably between about 4 and 40, and preferably between about 5 and 20. That is, the width of the spectrum detected by each pixel of the detector is less than xc2xcth and preferably less than ⅕th the excitation source wavelength deviation.
In a typical embodiment of the present invention, the computer can be a general-purpose personal computer programmed to contain a library of digital maps of various materials of known composition. In the preferred embodiment, the digital maps consist of a second-derivative vector of the spectrographic output as a function of Raman shift wavelength smoothed to exclude the short wavelength pixel noise. The resulting second derivative is treated as an n-dimensional vector where n is the number of pixels in the spectrographic output. The computer is programmed to compare the information taken from a sample of unknown composition with the library by computing a vector dot product of the vector of the sample with each specimen vector stored in the library. The dot product constitutes a scalar evaluation of coincidence between each pair of vectors. Based on this computation, the computer can be programmed to select the best fit by merely retaining the identity of the known material having the highest value for the dot product, and displaying that identity. Where the library contains vectors of a number of very closely related materials, the computer can be programmed to retain all library-stored vectors for which the vector dot product was greater than a specified value. The computer can then conduct a review of additional criteria of the sample vector that could discriminate the retained library vectors to select the best fit. This feature, when employed by even conventional personal computers having operating speeds in the order of 300 MHz, provides an accurate identification of a wide variety of materials, typically in only 1 or 2 seconds. Where very rapid sample identification in desired, for example in automated systems, dedicated digital signal processing equipment can be employed rather than general purpose personal computers.
By offering a simple means to overcome the difficult problem of identification of materials for recycling, the present invention seeks to help make manufacturers more receptive to the idea of including recycled content in their products and give purchasers more confidence in the quality of those products. The same system programmed with a slightly different library of reference materials can enable companies to monitor a wide variety chemical constituents during manufacture and permit the assay waste stream flows into the environment for any unwanted contamination. Law enforcement agencies are also able to use the system to identify the presence of illicit drugs in the field without affecting the chemical make-up of the sample or detracting from the amount of material tested.
Additional features and advantages of the invention will become apparent to those skilled in the art upon consideration of the following detailed description of the preferred embodiment exemplifying the best mode of carrying out the invention as presently perceived.